1. Field of the Invention
The present invention relates to a power output apparatus, a method of controlling a power output apparatus, and a driving system with a power output apparatus incorporated therein. More concretely, the present invention pertains to a power output apparatus for outputting power generated by an engine to a drive shaft at a high efficiency and a method of controlling such a power output apparatus, as well as a driving system with such a power output apparatus incorporated therein.
2. Description of Related Art
In known power output apparatuses mounted on a vehicle, an output shaft of an engine is electromagnetically linked with a drive shaft, which connects with a rotor of a motor, by means of an electromagnetic coupling, so that power of the engine is transmitted to the drive shaft (as disclosed in, for example, JAPANESE PATENT LAYING-OPEN GAZETTE No. 53-133814). The electromagnetic coupling of the power output apparatus transmits part of the power output from the engine as a torque to the drive shaft via electromagnetic connection, and supplies electric power regenerated by sliding motions of the electromagnetic coupling to the motor and secondary cells, which are connected in parallel with the electromagnetic coupling. When the torque transmitted to the drive shaft by means of the electromagnetic coupling is insufficient, the motor applies an additional torque to the drive shaft with the electric power regenerated by the electromagnetic coupling or the electric power released from the secondary cells. The motor works as a generator when a braking force is applied to the drive shaft, so as to regenerate the energy of rotational motion of the drive shaft as electrical energy and store the regenerated electrical energy in the secondary cells.
In the conventional power output apparatuses, however, problems, such as an extremely low energy efficiency of the whole apparatus or an extremely poor emission, arise in some cases. The power output apparatus can implement torque conversion of all the power output from the engine with the electromagnetic coupling and the motor and output the converted power to the drive shaft. The electromagnetic coupling and the motor carry out energy conversion of the power or energy expressed as the product of the torque and the revolving speed of the engine into the power or energy expressed as the product of the torque and the revolving speed of the drive shaft under the condition of constant energy. on the assumption that the conversion efficiency is an ideal state (that is, the value `1`), the object of torque conversion is to make the power output from the engine equal to the power output to the drive shaft. The engine may accordingly be driven at any driving point (defined by the revolving speed and the torque) that can output energy identical with the power. Without the active control of the driving point of the engine, this often causes the engine to be driven at driving points of low energy efficiency or poor emission.